Recent wide use of Local Area Networks (LANs) is increasing the demand for connecting a LAN with another or connecting a LAN with a Wide Area Network (WAN). For example, there can be considered a case where LANs geographically distant from each other are connected through a public network to transfer electronic mails and data files. A router is used as a device connecting a Local Area Network with another through a Wide Area Network. The router analyzes the header of a received data packet for routing and transfers the packet to the destination Local Area Network through the Wide Area Network.
A conventional packet transmission system in such a router is designed to disconnect the public circuit when a predetermined time interval passes after the transmission of data packets between the LAN and the public network has come to an end. That is, the system is provided with a timer in which a time interval is set in advance. When the packet transmission ends, the timer is started, and when the set time has elapsed, the public circuit is disconnected according to a output signal of the timer. The timer is set for a fixed time interval selected by the user at system installation.
In the conventional packet transmission system, however, the timer is usually set in advance for a relatively large value so that the public circuit is not disconnected in the middle of operation in any service. For example, in the case of a conversational processing application where the higher level protocol is a remote terminal protocol such as TELNET and a data packet is transmitted each time the operator taps the keyboard, the timer must be set for a enough time interval that the public circuit is not disconnected in the middle of operation. On the other hand, in consecutive processing application of a file transfer protocol (FTP), a short timer set value is sufficient. Therefore, in the conventional system in which the timer set value is fixed, the timer is set for a time interval long enough to accommodate a conversational protocol as TELNET.
In this way, in a case where the higher level application is an FTP which transfers packets at high speed, a problem arises in that public-circuit connection continues long after the transmission has ended. As a result, the public circuit is prevented from efficient use. From the view point of the user, it adversely affects service charges.
An object of the present invention is to provide a packet transmission system which enables a public circuit to be used efficiently while saving on service charges.